Exhaust gas discharged from a lean-burn engine contains various harmful substances derived from fuel or combustion air. Such harmful substances include a hydrocarbon (HC), a Soluble Organic Fraction (it may also be referred to as SOF), soot, carbon monoxide (CO), a nitrogen oxide (NOx) and the like, and regulations on discharge amount of these harmful substances have been tightening year by year. As a purification method for such harmful substances, a purification method by contacting exhaust gas to a catalyst has been practically applied.
In addition, in such a lean-burn engine, there has been investigated suppression of generation amount of the harmful substances by controlling kind, supply amount and supply timing of fuel, amount of air or the like. However, a conventional catalyst or control method has not been possible to purify exhaust gas in a satisfactory level. In particular, because of easy discharge of a nitrogen oxide in a lean-burn engine, as well as in view of ever tightening of regulations thereof, existing purification technology of NOx is difficult to suppress discharge of the harmful substances, in the case of a diesel engine to be mounted on an automobile, due to always changing operation condition thereof.
As the one using a catalyst among technology (denitration technology) for purifying NOx, there has been known a technology for reductive denitration by making exhaust gas comprising NOx contacted with a selective reduction catalyst having vanadium oxide, zeolite or the like, as a main component, under the presence of an ammonia (NH3) component, as a selective reduction method, or Selective Catalytic Reduction (hereafter may be called SCR).
In this SCR, where the NH3 component is used as a reducing agent, NOx is finally reduced to N2 mainly by the following reaction equations (1) to (3):4NO+4NH3+O2→4N2+6H2O  (1)2NO2+4NH3+O2→3N2+6H2O  (2)NO+NO2+2NH3—2N2+3H2O  (3)
In denitration in exhaust gas, in the above denitration reactions (1) to (3), molar ratio of NH3/NOx is enough to be 1.0 theoretically, however, in the case of transitional engine operation condition in operation of a diesel engine, or in the case where space velocity, temperature of exhaust gas, and temperature of the catalyst surface are not suitable, in order to obtain sufficient denitration performance, there may be the case where ratio of NH3/NOx of NH3 to be supplied should be increased inevitably, resulting in leakage of unreacted NH3, therefore inducing risk of secondary pollution such as new environmental contamination or the like has been pointed out. Hereafter, NH3 leakage may be referred to as slip or NH3 slip.
In such a denitration catalyst system, NH3 gas may be used as the reducing component, however, NH3 itself has irritating odor or harmful property. Therefore, there has been proposed a system for adding urea water, as the NH3 component, from the upstream of a denitration catalyst, generating NH3 by pyrolysis or hydrolysis, and having this acted as a reducing agent to exert denitration performance.
Reactions for obtaining such a NH3 by decomposition of urea are as the following (4) to (6):NH2—CO—NH2→NH3+HCNO  (4; pyrolysis of urea)HCNO+H2O→NH3+CO2  (5; hydrolysis of isocyanic acid)NH2—CO—NH2+H2O→2NH3+CO2  (6; hydrolysis of urea)
Urea is supplied by spraying as urea water from the upstream of the SCR catalyst. As described above, because the one which contributes to reductive purification of NOx is mainly NH3, a reaction of NOx in the SCR catalyst is influenced by decomposition efficiency of urea. Low decomposition efficiency of urea not only decreases efficiency of NOx purification but also increases use amount of urea, and could induce NH3 slip caused by unreacted urea.
As for such NH3 slip, for oxidative purification of slipped NH3, it was necessary to arrange an oxidation catalyst at the later stage of the SCR catalyst. However, arrangement of such a catalyst for purification of slipped NH3 leads to increase in cost, and it was difficult to secure mounting site of the catalyst, in particular, in an automobile.
In addition, increase in amount of slipped NH3 requires high oxidation capability to the catalyst, and it was necessary to use a large amount of a valuable noble metal such as platinum, which is an activated specie.
In purification of NOx by the NH3 component, the reaction is promoted under atmosphere containing NO and NO2, each in an amount of roughly half, as in the above formula (3) (NON PATENT DUCUMENT 1). However, most of the NOx component discharged from a lean-burn engine is nitrogen monoxide (NO) (PATENT DUCUMENT 2). Therefore, for efficient purification of NOx, there has been proposed to arrange an NO oxidation means in a flow passage of exhaust gas, in order to increase concentration of the NO2 component in exhaust gas (PATENT DUCUMENT 2).
There has also been proposed a method for purifying harmful fine particle components and NO at the same time by a single catalyst system, by utilization of such an NO oxidation means. One of them is arrangement of the oxidation catalyst, a filter, the SCR catalyst in a flow passage of exhaust gas, in this order, and spraying of an ammonia component at the front stage of the SCR catalyst (refer to PATENT LITERATURE 3).
In addition, because exhaust gas from a gas turbine or a gas engine has high temperature and high SV (space velocity), removal of NO under such conditions has been a problem for catalytic action of the selective catalytic reduction (SCR). As a catalyst for performing selective catalytic reduction of a nitrogen oxide using ammonia at an exhaust temperature over about 300° C., there has been proposed the SCR catalyst comprising the first component containing zeolite, the second component composed of each substance such as cerium, iron, copper, or a mixture thereof, and an oxygen storage substance (refer to PATENT LITERATURE 1). As this SCR catalyst, there has been exemplified in Example, a “cerium-mixed wash-coat catalyst” using a material comprising alumina, mixed zeolite, and a Ce/Zr-type oxide, and has been reported that high NO removal efficiency was obtained at such a high temperature of 550° C.
In exhaust gas from a diesel engine, as has been described in PATENT LITERATURE 1, space velocity may change in a wide range from 1 khr−1 to 150 khr−1. PATENT LITERATURE 1 has confirmed denitration efficiency of the SCR catalyst at a relatively low space velocity of 15 khr−1 to 25 khr−1, however, it is considered that denitration efficiency decreases in relatively high space velocity over this range.
In addition, in recent years, there has been such a tendency that the number of catalysts to be used in an exhaust gas purification system of a lean-burn engine increases with ever strengthening exhaust gas regulations. In particular, in the case of an automobile, which is a mobile internal combustion engine, a problem of mounting space of an apparatus, or low fuel efficiency/high output characteristics have been required to be solved. In view of these requirements, weight reduction and compact sizing per one catalyst has been required, as well as reduction of pressure loss has been necessary. PATENT LITERATURE 1 has not been performed investigation on these problems, and thus cannot be said practical as an exhaust gas purification catalyst.